This project fell under my aegis during the last year before Gary passed away, when I was acting as organizer for the Yggsburgh town expansion modules, and when Gary had tabbed me to oversee the five or six Castle Zagyg adjunct modules, as Scotty can attest to. Bill Silvey was indeed its developer, but the module was incomplete per Gary's reckoning. There were elements he required that were missing from the whole, and other revisions. It never went beyond that stage, unfortunately. Another thing is this: Gary was never too keen on it being a CZ-related project. He wanted it to be its own entity, with a possible connection to CZ, but not necessarily part of the same branding. I don't know what, if any, future this adventure has.
I may still have an e-mail from when I was working on this where Gary mentions not wanting it to be strongly connected to the dungeons. He gave a reason, but I can't remember what it was. At the time, the CZ project was still up in the air, so I never gave it much thought. The intro did tie it to the dungeons, but in a light way, similar to the EX modules. The background I wrote had deeper ties to the setting.